The invention relates to honeycomb core layers such as are used in sandwich materials for packaging and structural applications, as well as to processes and apparatus for producing these honeycomb cores.
In the aerospace industry, honeycomb cores have preferably been used for many decades as core material for sandwich panels and boards that are resistant to buckling and bending. These honeycomb cores, which are mostly hexagonal or over-expanded, consist predominantly of aluminum or phenolic-resin impregnated aramide fiber paper and are usually produced in the expansion process. A sandwich structure having two, usually adhesively bonded cover layers provides extremely high stiffness/weight and strength/weight ratios. The interest expressed by other large branches of industry in lightweight sandwich core materials with good weight-specific material characteristics is continually growing, so that in the meantime more than half the honeycomb core materials are being used in other sectors.
The use of honeycombs for packaging, in automobile and comparable markets requires fast and continuous production of the honeycomb core layer, in order that a product which is competitive with corrugated board and other inexpensive materials can be produced.
A sandwich with a honeycomb core has high specific compressive strengths in the material plane, because of the optimum, virtually orientation independent, support of the covering layers. As compared with a sandwich with a corrugated core (for example corrugated cardboard), better edge compression resistances and flexural rigidity values can be achieved, in particular in the machine direction. As a result, significant savings in weight and material are possible. The pressure characteristics are also considerably better at right angles to the plane of the material because of the perpendicular, mutually supporting cell walls. In addition, a sandwich with a honeycomb core has a better surface quality, which is important in particular for printing packaging materials. Because of these advantages and the increase in demand for inexpensive sandwich cores, numerous efforts have been taken in the past to reduce the high production costs of honeycomb cores.
Many processes are known in which individual material strips or a continuous material web is adhesively bonded at alternating locations and then expanded (U.S. Pat. No. 4,500,380 Bova, DE 196 09 309 Hering, U.S. Pat. No. 4,992,132 Schmidlin, U.S. Pat. No. 5,334,276 Meier). Such processes are already used for the partially automated production of paper honeycombs with cell sizes above about 10 mm for internal packaging, edge and corner elements and also for pallets. The necessary forces and the material stresses during expansion place high requirements on the adhesive and the adhesive bonding of the cell walls. By means of pre-embossing the fold lines, these forces can certainly be reduced but the regularity of the honeycomb geometry suffers from the expansion process, in particular in the case of paper honeycombs with small cell sizes. The internal stresses and the necessary expansion forces are increased considerably in the case of smaller cell sizes. For this reason, these processes are increasingly problematic for smaller cell sizes and more difficult to automate. In addition, the production speed is limited by the required cross-cutting of the web.
Likewise, many processes are known in which individual, corrugated or trapezoidal material webs or strips are adhesively bonded in an offset manner (U.S. Pat. No. 3,887,418 Jurisisch, U.S. Pat. No. 5,217,556 Fell, U.S. Pat. No. 5,399,221 Casella, U.S. Pat. No. 5,324,465 Duffy). Technical implementation to obtain a continuous process with a high production speed is difficult in the case of these processes, because of the necessary positioning and handling of the individual material webs.
Furthermore, processes are known in which corrugated cardboard is processed to form honeycomb cores. In one process, corrugated cardboard is used in the cell walls of honeycomb cores (U.S. Pat. No. 4,948,445 Hess). In this case, individual sheets of corrugated cardboard with flutes running in the production direction are supplied and short cross-cuts going through the entire thickness of the corrugated cardboard are introduced. Therefore, following folding in the production direction and expansion, corrugated cores with relatively large cell sizes and relatively thick cell walls are produced. The process is principally the same as the expansion process with a continuous material web.
Furthermore, honeycombs and processes are known in which a corrugated cardboard web (U.S. Pat. No. 3,912,573 Kunz) or an individual corrugated web (WO 91/00803 Kunz) with the flutes transverse to the production direction is cut into strips. After the web has been cut up, a honeycomb core layer is then produced by bonding the individual strips adhesively to one another. This process requires a certain size of the individual strips or special positioning tapes, in order that their handling is still ensured. Because of the size of the strips, the web width is reduced considerably following the rotation of the strips. In order not to obtain too small a width of the honeycomb core layer, the strips are cut off in a further production step and adhesively bonded to form a honeycomb block, which is then conveyed considerably more slowly transversely to the production direction. For small honeycomb heights, this honeycomb block has to be cut up, if appropriate. The honeycombs produced by such a process also have individual straight strips between individual corrugated or trapezoidally shaped cell wall strips. Such reinforced honeycombs are also known from manual production via a block (WO 95/10412 Darfler). There, the individual flat layers are placed between the individual corrugated layers and adhesively bonded to them.
Honeycombs and processes for their production are also known in which a continuous material web, following the introduction of cuts, is initially corrugated or formed trapezoidally before the connected cell walls are folded against one another and adhesively bonded (WO 97/03816 Pflug). In order to achieve a saving in material in packaging applications, in particular as compared with corrugated cardboard, a very lightweight paper (40 g/m2 to 80 g/m2) is to be preferred. When corrugating these low grammages, it is advantageous to stabilize the flute immediately after it has been formed by bonding it onto a web. In particular, in the case of corrugation transversely to the production direction, as is common in the production of corrugated cardboard at speeds up to 350 m/min, a covering layer (a liner, as it is known) has to be bonded on immediately. The corrugated web on its own cannot absorb the tensile stress necessary for the rapid conveyance of the material web.
Furthermore, processes and apparatus are known for introducing slits into corrugated cardboards (U.S. Pat. No. 5,690,601 Cummings). These slits are made along the flutes of individual sheets of corrugated cardboard (in the transverse direction to the actual production direction of the corrugated cardboard) in order to permit a defined folding. In this process, folding is carried out towards the cut so that the latter is closed.
The invention is based on the object of specifying a honeycomb core layer, a process and an apparatus which permit the continuous production of honeycombs with relatively small cell sizes at a production speed comparable with the production of corrugated cardboard. In addition, good surface quality as well as reliable and quick attachment of the covering layers is desired.
The object set is achieved on the basis of the measures of claims 1, 7 and 14 and with the aid of the intermediate product as claimed in claim 23, and developed further by further features of the subclaims.
In the invention, a corrugated or trapezoidal material web having at least one, but preferably having two, covering layers is supplied. This can be corrugated cardboard or else a plastic fibre composite or metal corrugated core board. In addition, a web having a plurality of corrugated cores, for example a double-flute corrugated cardboard (BC flute, AA flute) can be used. The cover layers preferably also consist of very thin material (weight per unit area between 60 g/m2 and 100 g/m2) and the corrugated core layer consists of material up to twice the thickness since the covering layers in the preferred variant of the folded honeycomb are laid doubled. In this case, very low demands are placed on the quality of the cover layers, and also on the thickness tolerance and surface quality of the corrugated core web, since these factors have little influence on the surface quality of the end product.
The thickness of the corrugated cardboard web determines the size of the honeycomb cells. In order to support the cover layers, cell sizes of 4.7 mm (A flute) or, at very low weights per unit area, 3.6 mm (C flute) are adequate since the flat corrugated core cover layer strips provide an additional support and reduce the risk of dimpling of the cover layers in the cells. However, corrugated cores with smaller and larger cell sizes can also be produced from corrugated core webs with smaller and larger heights of the flute (e.g. K flute).
According to one embodiment of the invention, the multi-layer web is firstly provided in the conveying direction with a large number of continuous fold lines on the underside and the upper side. The fold lines can be introduced, for example, by means of pressing or longitudinal cutting of the web. The cuts do not quite cut through the web in the thickness direction but in each case leave a continuous cover layer (or the cover layer and the peaks of the flutes). The cuts on the upper side are in this case located as accurately as possible halfway between the cuts on the underside. The irregularities in the cover layers, which are normal in corrugated cardboard, and the different cutting forces between the peaks of the flutes can lead to the cover layer being partly or wholly cut through at individual points. This is entirely desirable, provided that the corrugated core strips still remain connected in the transverse direction. The necessary folding force can be reduced by this slight cutting or perforation of the cover layers or an additional pre-embossing of the fold lines. The corrugated core strips can also first be completely cut through and, at the same time or immediately thereafter, adhesively bonded together by means of adhesive foils. This material may be easier to bend and to fold, as compared with the material of the web. Therefore, the combination of words xe2x80x9cformed in one piecexe2x80x9d not only includes corrugated strips which are connected to one another by a cover layer, but also separate corrugated strips which are connected to one another by adhesive foil. The ratio between the width and the height of the connected corrugated core strips is preferably in the range from 0.5 to 2.0.
The connected corrugated core strips are then in each case rotated through 90xc2x0 in such a way that the cuts open and the connected cover layers of adjacent strips are folded through 180xc2x0. Since the strips are connected, no alignment in the thickness or longitudinal direction is necessary. The strips lie planar next to each other with the connected cover layers and form the folded honeycomb. They can be adhesively bonded, joined in any other way or joined by the new cover layers only when the latter are bonded on. The application of the adhesive can then be carried out by means of rollers, nozzles or brushes, an application which constantly applies a relatively low amount of adhesive being preferred. When use is made of a corrugated core web having two cover layers, the corrugated core strips are significantly more stable than only with one cover layer, and can be adhesively bonded with some pressure. Possible deformations of the corrugated core, which often impair the surface quality in the production of corrugated cardboards, takes place here in the width direction and has no influence on the surface quality and thickness tolerance of the folded honeycomb.
The flat corrugated core cover layer strips, which are vertical in the honeycomb, are able to accommodate the tensile stresses in the production direction and permit a fast transport of the material web. They subsequently increase the shear and compression characteristics of the honeycomb, so that all the material of the corrugated cardboard is utilized in the honeycomb core folded from it.
In order to produce a honeycomb board material, new covering layers can be adhesively bonded continuously onto the honeycomb core layer immediately after the honeycomb production. In this case, the high compressive strength of the honeycomb is very useful. Good attachment of the cover layers to the honeycomb can be achieved by slight defibering of the edges during the introduction of the longitudinal cuts. In addition to the edges of the corrugated core layer, the small side faces of the folded corrugated core cover layer strips are additionally available for the attachment of the cover layers.